Pokémon Lv. X were introduced in the Diamond & Pearl series as the successor to Pokémon-ex. Pokémon Lv. X are Levelled Up versions of the Pokémon and so you put the card on top of the Pokémon. As such, you need to have that Pokémon already. However, despite that, it still counts as a Pokémon card so including Lv. X, you still cannot have more than 4 of any one Pokémon in your deck.

Pokémon Lv. X have the ability of letting you use the attacks & powers of the Pokémon you levelled up from. This makes them incredibly versatile and worth it.